What Are the Planets in Order?

The four inner planets are Mercury, Venus, Earth and Mars. They are small, rocky and close together. The four outer planets are Jupiter, Saturn, Uranus and Neptune, and they are enormous, made mostly of gas and ice, and spread far apart.

The dividing line is the asteroid belt, and the reason for the split goes back to how the solar system formed. Close to the young Sun it was too hot for ice to survive, so only rock and metal could condense. Further out, ice was available in vast quantities, letting planets grow far larger.

Mercury is the smallest, closest and has almost no atmosphere, so its surface swings between extreme heat and cold. Venus is nearly Earth's size but has a thick carbon dioxide atmosphere and a runaway greenhouse effect that makes it the hottest planet at around 460 degrees Celsius.

Earth is the only one known to have liquid water on its surface and life. Mars is about half Earth's diameter, cold and dry now, but with clear geological evidence that liquid water flowed there in the past.

Jupiter is more massive than everything else orbiting the Sun combined, with a storm system, the Great Red Spot, larger than Earth. Saturn is famous for rings made of countless ice and rock particles, and is so low in density that it would float in a sufficiently large ocean.

Uranus and Neptune are often called ice giants because they contain more water, ammonia and methane than Jupiter and Saturn. Uranus is tipped almost on its side, so it rolls around its orbit rather than spinning upright.

A few facts that stick:

• •A day on Venus is longer than its year
• •Mars has the tallest volcano in the solar system, Olympus Mons
• •Jupiter has more than 90 known moons
• •Neptune has the fastest winds recorded anywhere in the solar system

Pluto was reclassified as a dwarf planet in 2006. The trigger was the discovery of other similar sized objects in the outer solar system, which meant either admitting many more planets or tightening the definition.

The definition adopted requires a planet to orbit the Sun, be round under its own gravity, and have cleared other objects from its orbital neighbourhood. Pluto meets the first two but shares its region with many other bodies, so it fails the third.

The five planets visible without a telescope, Mercury, Venus, Mars, Jupiter and Saturn, have been known since antiquity, and Earth joined the list once the Sun-centred model was accepted. Uranus was the first planet ever discovered, spotted by William Herschel in 1781.

Neptune was found differently and more impressively. Astronomers noticed that Uranus was not following its predicted orbit, calculated where an unseen planet would have to be to explain the discrepancy, and found Neptune within a degree of the predicted position in 1846. It remains one of the most striking confirmations of Newtonian gravity ever made.

Diagrams of the solar system nearly always squash the planets together so they fit on a page, which hides how empty space really is. Astronomers often measure distances in astronomical units, where one unit is the average distance from Earth to the Sun, about 150 million kilometres. On that scale, Mercury is about 0.4 units from the Sun, Mars about 1.5, Jupiter about 5.2, Saturn about 9.5, Uranus about 19 and Neptune about 30. The gap between Saturn and Uranus alone is larger than the whole distance from the Sun to Saturn.

Light travel time gives another sense of scale. Sunlight takes a little over eight minutes to reach Earth, but around four hours to reach Neptune. That delay matters for space missions, because a radio command sent to a distant spacecraft takes hours to arrive and hours more for any reply to come back, so probes must be able to carry out many tasks on their own. If you drew Earth as a grain of sand, the planets would be scattered across an area many times larger than a football pitch.

Many people remember the order with a sentence whose words start with the same letters as the planets. A popular modern version is 'My Very Educated Mother Just Served Us Noodles', for Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Older versions ended with something like 'Nine Pizzas', and had to be rewritten after Pluto was reclassified. Making up your own sentence often works even better, because a strange or funny image you invented yourself is easier to recall than one you were simply given.

The order from the Sun is not the same as the order of distance from Earth. Venus comes closest of any planet at its nearest approach, so many people assume it is our closest neighbour. Yet when you average the distance over time, Mercury wins. Venus spends long periods on the far side of the Sun, very far away, while Mercury's small orbit keeps it relatively close to the Sun and therefore never extremely far from us. By the same reasoning, Mercury is on average the closest planet to every other planet, including Neptune.

The word planet comes from the Greek for wanderer. Ancient sky watchers noticed that a handful of bright lights drifted against the fixed patterns of the stars, and they counted seven of these wanderers, including the Sun and the Moon. Their names reached English through Latin, so the five visible planets carry the names of Roman gods: Mercury, the swift messenger, for the fastest-moving planet; Venus, goddess of love and beauty, for the brightest; Mars, the god of war, for the red one; Jupiter, king of the gods; and Saturn, an older god of farming and time.

Those seven wanderers also gave us the days of the week. Saturday is Saturn's day, alongside Sunday and Monday for the Sun and Moon, and in French and Spanish the weekdays still carry Mars, Mercury, Jupiter and Venus, as in mardi and viernes. English swapped in Norse gods thought to be their equivalents, so Thursday belongs to Thor in place of Jupiter. When Herschel found Uranus he wanted to honour King George III with the name Georgium Sidus, but many astronomers preferred a name from mythology, and Uranus, after the Greek god of the sky, eventually won.

The order of the planets is also an order of speed. In 1619 Johannes Kepler published the rule that a planet's orbital period squared is proportional to its average distance from the Sun cubed. Measured in years and astronomical units, it becomes simply period squared equals distance cubed. Mars sits about 1.52 units from the Sun, and 1.52 cubed is about 3.5, whose square root is roughly 1.87, so a Martian year lasts a little under two Earth years. For Jupiter at 5.2 units, 5.2 cubed is about 141, giving a year of about 11.9 Earth years, close to the measured value.

Neptune, at about 30 units, needs roughly 165 years to go round once, so it completed its first full orbit since its discovery only in 2011. Mercury, at about 0.39 units, finishes a lap in 88 days. Outer planets are slower for two reasons at once: their orbits are longer, and they move more slowly along them because the Sun's pull is weaker out there. Earth travels at about 30 kilometres per second, Neptune at only around 5. Isaac Newton later showed that Kepler's rule follows from gravity, and the same reasoning lets astronomers weigh a planet by timing the orbits of its moons.

The planets in order are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, split into small rocky worlds close in and giant gas and ice worlds further out, with the split explained by the temperature of the early solar system.